Thermal printing head

ABSTRACT

A high density thermal printing head includes resistive heating elements on a layer of silicon which is epitaxially grown on a sapphire substrate. Isolation diodes are diffused into the silicon layers and in accordance with various embodiments of the invention, transistor decoding circuitry, logic control and other circuitry may also be diffused therein. To facilitate the arrangement of the heater elements in an array without the necessity of numerous layers of metallic interconnections, portions of a layer in the silicon are doped with impurities to serve as highly conductive interconnections between various elements on the head. These interconnections are crossed beneath other metallic interconnections on the surface of the silicon to provide the electrical crossings necessitated by the physical arrangement of elements on the head. In some embodiments, the diodes and/or transistors are diffused into mesa-like regions of the silicon in direct communication with heater elements overlayed on the mesas.

United States Patent 119 I Primary Examiner-(3. L. Albritton I i Attorney, Agent, or Firm-Ronald E. Grubman Bohorqueiz et al. i 1 Dec. 3, 1974 i 1 THERMAL PRINTING HEAD [75] Inventors: Jaime H. Bohorquez, Escondido; [57] ABSTRACT 22;? MacNell powayigbot'h of A high density thermal printing head includes resistive v V heating elements on a layer of silicon which is epitaxil l Assigneer flewltt-PackardCompany, P810 ally grown on a sapphire substrate. Isolation diodes Alto, Califare diffused into the silicon layers, and in accordance [22] Filed: Feb 1, 1974 with various embodiments of the invention, transistor decoding circuitry, logic control and other circuitry PP N08 438,883 may also be diffused therein.

, To facilitate the arrangement of the heater elements in [52] U.S. Cl 219/216, 219/543, 317/235 Q, an array without the necessity of numerous layers, of 346/76 R metallic interconnections, portions of a layer in the [51] Int. Cl. H05b 1/00 silicon are doped with impurities to serve as highly [58] Field-0f Search 219/216, 543; 29/576 OC, conductive interconnections between various elements 29/576 E, 580; 317/235 Q; 346/76 R on the head. These interconnections are crossed 1 beneath other metallic interconnections on the surface .[56] References Cited of the silicon to ,provide the electrical crossings UNITED STATES PATENTS necessitated by the physical arrangement of elements 3,598,956 8/1971 Cady, Jr 2l9/2l6 h head In F F 'q and/0r 3-609.294 9H9, Cady hm 219/543 X transistors are diffused 1nto mesa-like regions of the 3,700,852 10/1972 Ruggiero 219 543 x slllco" direct commumcatlon heater elements overlayed on the mesas.

5 Claims, 10 Drawing Figures Column 1 en's-MA Column 2 II/ I n:- a

O I Column, n

PAIENIEL 3W SHEU 10F 4 Row 10* Y RowZO Row n RowlCl Column n Pmm'imin 31914 3,852,563

sum 2 or 4 igure 3 PATENTEL BEE 31974 SHEET 30F 4 IQB 1 THERMAL PRINTING HEAD BACKGROUND OF THE INVENTION Thermal printing heads are currently known in which resistive heating elements and silicon semiconductor elements are bonded onto an alumina substrate. This typeof device requires large numbers of external bonds to the printing head as well as large numbers of internal wire bonding among the various semiconductor and resistive printing elements. Consequently, the assembly yield for the production of such printing heads tends to be very low. Even more importantly, theseprior art printing heads cannot be used for high density printing, since an even greater number of external bonds would have to be made .to the head, an almost insurmountable task if the desired packing density of the heater elements is sufficiently high. v

A different kind of thermal printing head known in the prior artis disclosed in U.S. Pat. No. 3,515,850, Thermal Printing, Head with Diffused Printing Elements, issued to Richard C. Cady, Jr. on June 2, 1970. This thermal printing head utilizes a single crystal sapphire substrate upon which a layer of silicon is epitaxiallydeposited. Anarray fof resistive heating elements and isolationdiodes are diffused into the, semiconductor layer..The device requires the use of numerous layers of metallic conductors interposed between various semiconductor layers in order to make electrical interconnections among the elements. This'structure appears to be very complicated and might lead to thermal stresses fromone layer to another, thereby reducing the reliability of the device. Furthermore, the printing head disclosed in the above-mentioned U.S. patent does not include any transistor decoding elements, so

electrical connections which need be made to the print head. However, in making electrical interconnections between the various elements on the print head, some connections will inevitably have to cross under or over other connections. According to the present invention, the required crossings are achieved without the use of multiple layers of conducting elements. Instead, a thin portion of the silicon layer adjacent to the sapphire substrate is doped with impurity atoms to form highly conductive regions which may conduct current between the diodes, transistors, and heaters on the printing head. Wherever required, this conductive layer serves to cross some electrical connections beneath other metallic connections which are formed on the top surface of the silicon layer.

In accordance with various embodiments of the invention, the diodes and/or transistors on the printing head are diffused into raised areas, or mesasof the sili-' printing head must be made, the disadvantages of which are described above. r r

It would, therefore, be desirable to have a high density thermal/printing head which requires only a relatively few external bonding-connections to'the head.

The desired printing head would also include only a single layer in which metallic conductors were disposed between elementson th e head itself. Preferably, transistor decoding logicwould itself bean integral par'tof the'printing head. I

SUMMARY OF THEY-INVENTION,

According to theillustrated preferred'embodiments, the presentv invention-provides a'high density closely packedthermal printing head in whicha numberof reetched and doped to produce diode isolation circuitry and/or transistordecoding circuitry. Also included on the sapphire substrate are a number of resistive element'swhich serve as printing elements when heated. Electrically the heatingelements are arranged as a matrix so, that any particular heater may be activated by electrically activating one row, andone column of the matrix. Physically, however, the heating elements are positioned in alinear array to function as a print head.

con. The heater elements are then directly overlayed on these raised portions, so that the heater elements and associated circuitryare in direct communication,

thereby further reducing the number of interconnections required.

strate. I

er-transistor structure including an electrical, cross- -under using-a doped highly conductive layer in a silicon DESCRIPTION OF THE INVENTION In FIG. l'thereis schematically illustrated a matrix array of isolation diodes and heating elements used for printing. By addressing a particular row' andcolumn of the array anyone of the heating elementsm'ay be individually activated. A typical'printing, orfheating element is labelled 11- while itsassociated isolation diode is labelled l3. a

To function as a linear printing head, the heating elements must be physically oriented in a linear array. Such an array is schematically indicated by FIG. 2 in which printing element 11 and isolation diode 13 are identified by' the same numbers as inFIG. 1'. In a high l l I yield,.it-is preferable to produce several smaller-print- This matrix arrangement greatly reduces the numberofing elements each containing a fewer number of printingelements. The smaller units are then combined to make the final printing head. However, a difficulty in using a number of smaller elements fastened together is that uneven spacing between the resistor elements is likely to result from'poor edge-to-edge abutment between the smaller elements. To alleviate this problem, the present invention uses a substrate of single crystal sapphire. As is known in the art, sapphire may be highly polished so that by polishing adjacent edges of individual pieces, good continuity and even spacing between resistor elements may be achieved when smaller units are fastened together to form. a larger printing head. Various embodiments of a thermal printing head using a sapphire substrate are described below.

In FIG. 3, there is illustrated a preferred embodiment of the invention including a sapphire substrate 15 of the invention, an isolation diode may be fabricated by doping silicon layer 21 with phosphorus to an'impurity concentration of about 10 impurity atoms/cc. in

a region about 1 p. thick directly adjacent to sapphire substrate 15. This region constitutes an N highly conductive region labelled 25. Immediately above N region 25 is a region of about 4 p. thick constituting an N layer 27 with an impurity concentration of about 5 X atoms/cc. Arsenic doping is suitable for fabrication of this layer. The anode end of the isolation diode which is about-400 pi. thick. Generally, thicknesses-of sapphire substrate 15 in'the range 150 p. to 1000 p. are

suitable. Also illustrated in FIG. 3 is a thin film resistive In FIG. 5,there is illustrated a sapphire substrate 15 on which-a layer of silicon has been-epitaxiallygrown.

In thepreferredembodiment of FIG. 5,"the epitaxially grown silicon takes the form of a raised island 21,

thickness about'2 u to 10 1.4.. The raised island may be produced elg. by first epitaitially. 'growing"a uniform layer of silicon overthe entire sapphiresubstrate, and then performing an anisotropic etch (i.e. an orientation dependent etch)-with a chemical such as alcohol hydrazine. As is'known in the art, anisotropic etching along the 100 orientation of the crystal-plane will etch faster inl'one cleivage planethan inanother Thus, the s ilicon will taketheform of a series of islands, of which'island v 21 is a typical example. In this embodiment of the invention, resistive printing elements ,llare deposited to conduct current to and is formed from a P- region 29 which has an impurity concentration of about 7 X 10 Boron doping may be used effectively to create P* region 29. A cathode region 31 is fabricated by dopingtlie silicon with a material such as phosphorus to produce an N* region 31 of impurity concentration about 10 atoms/cc. This N region 31 is in contact with an aluminum contact 19.

Another aluminum" contact also labelled 19 is in contact with P* anode 29. Inthe preferred embodiment an insulating region 33- is formed on silicon layer 21.

This layer may be, for example,ofSiO -and serves to insulateihe pair of metal contacts 19 from silicon mesa 21 in those areas where contact is not desired.-

onto the topsurface of silicon layer 21. Better contact of the printingelem'en'ts with sheets of [paper to be printed is thereby-obtained. Also illustrated are a pair of metallic contacts 19 to deliver current toand from printing elements 11. An insulatingilayer 23' which may be e.g.-Si 0, is shown as beinginterposed between printing element;-l l andsiliconlayer'21. It may be noted that in an'embo diment using a-raised silicon mesa, thermal. printing'might be pe'rformed by directly heating up thesiliconmesa contacting an exposed area of the.sili- In FIGS. 7A and 7B there are shown an isolation diode and a heater element integrated on one silicon mesa. Corresponding numbers indicatecorresponding elementsin both figures. In FIG. 7A, a diode similar to that described in connection withFlG. 6 is diffused into a silicon mesa 21. It may be seen that a P region 29 serving as an anode has beendi'ffused into the right; most side of the mesa in thisembodiment. An insulating region 33 similar tothat in FlG.j 6 is included, but has been extended from the right-most portion to the extreme left-most portion of themesaJA Ta N or a TaAl heater element 11 is formed on-insulating region 33. As described in connectionwith FIG. 6, an N" region 25'functions to conduct current to an N? cathode illustrated with wide cross-hatching, wh'ilea node re'- gion 29 and cathode region 31ar'e illustrated with narf row cross-hatching. In operation, current flows into heater element 11 through'ajmetallize'd contact 19A and exits through a .metallized contact 198 which is in physical proximity to anode 29. Current then flows through anode 29 and silicon 21 tov the highly condu'c}. tive': return path 25 (see FIG. 7A) under the silicon layer and, thence tocathoderegioniil andout the me- .tallic contact 19C.zy

I In accordance-with anothe rembodiment' of the-inn vention,transistor decoding logic circuitry is itself'inclucled on the printinghead. FIGS. SAand 8B illustrate a configuration in which a transistor iiszdoped into silicon layer 21 on sapphire substrate 15. The'transisto'r writing speed, there'being no need to heat up and cool down the bulk silicon. l I

-In FIG. 6 there is shown a single crystal-sapphire substrate 15. A silicon mesa 2 1 is epitaxially grown on the sapphire substrate as-d'escribed above in connection with FIG. 5. By suitable doping of silicon mesa 21,- a latincludesa collector region 37 whichis' an Ni region of impurity concentration about 10" atoms/ecu, which may be achieved by phosphorus d'oping. A'basegregion 39 is a? region dopedto an impurity concentration of about 10 atoms/cm, which may be suitably done by boron'doping. Another N region of impurityconcem tration of about 10 atoms/ccsfunctions as an emitter 41. A'SiO insulating layer 43 is on collector region'37 and serves to isolate thecollector region from a Ta N or TaAl heater element 11 positioned above insulating layer 43. In FIG. 8B the emitter, collector and base regions as well as the heater are indicated by corresponding numerals. The broadly cross-sectioned areas 19A-19D are indicative of metallic conducting lines on the surface of silicon layer 21. These conductors serve as contacts to the emitter 41, base 39, collector 37, and heater 11. In FIG. 8A there is shown also a region 45 which is an N* region of impurity concentration about atoms/cc. As may be seen in FIGS. 8A and 88 this highly conductive region 45 serves to electrically connect the emitter contact 198 with another metallic 7 contact 19D by crossing under a metallic base contact 19C. The use of highly doped N region 45 as a crossunder electrical connection is important in the construction of matrix-like printing heads, since it is very often necessary to have various electrical lines cross one another spatially. By utilizing a conductive layer 45 at the base of silicon layer 21 to cross under certain metallic contacts there is eliminated the necessity for multilayer metallic interconnections throughout the printing head. This cross-under technique may also be employed in any of the other embodiments of'the invention illustrated in the preceding figures. The integrated logic and decode structure as illustrated in FIGS. 8A and 8B allowsmaximum utilization of area to increase packing density of the elements on the substrate. -By

using transistors, diodes and heating elements in various combinations, a variety of logic control and drive functions can be accomplished directly. on the printing head; All of these functions may be achieved 'without the problem of external wiring to the various elements.

a sapphire substrate;

a body of semiconductor material epitaxially grown on the substrate;

a plurality of thin film heating elements on the body of semiconductor material; I

a plurality of semiconductor devices diffused into the body of the semiconductor material;

a plurality of metallic interconnections among the heating elements and the semiconductor devices; and

a plurality of highly conductive regions diffused in a single layer of the body of semiconductor material, said regions being interconnected among the heating elements and the semiconductor devices to serve as electrical connections therebetween.

2. A thermal printing device as in claim 1 wherein the plurality of thin film heating elements are of materials selected from the group consisting of Ta N and TaAl.

'ment associated with that diode, one of the anode and cathode of the isolation diodebeing in electrical contact with its associated-heating element.

5. A thermal printing head as in claim 3 wherein said plurality of semiconductor devices further includes a plurality of transistors and other diodes diffused into the body of semiconductor material to serve as decoding, logic control and drive circuitry. 

1. A THERMAL PRINTING DEVICE COMPRISING: A SAPPHIRE SUBSTRATE; A BODY OF SEMICONDUCTOR MATERIAL EPITAXIALLY GROWN ON THE SUBSTRATE; A PLURALITY OF THIN FILM HEAING ELEMENTS ON THE BODY OF SEMICONDUCTOR MATERIAL; A PLURALITY OF SEMICONDUCTOR DEVICES DIFFUSE INTO THE BODY OF THE SEMICONDUCTOR MATERIAL;
 2. A thermal printing device as in claim 1 wherein the plurality of thin film heating elements are of materials selected from the group consisting of Ta2N and TaA1.
 3. A thermal printing head as in claim 1 wherein said plurality of semiconductor devices comprises a plurality of isolation diodes, each diode being associated with one element of said plurality of heating elements.
 4. A thermal printing head as in claim 3 wherein each isolation diode is diffused into the body of semiconductor material in a region directly below the heating element associated with that diode, one of the anode and cathode of the isolation diode being in electrical contact with its associated heating element.
 5. A thermal printing head as in claim 3 wherein said plurality of semiconductor devices further includes a plurality of transistors and other diodes diffused into the body of semiconductor material to serve as decoding, logic control and drive circuitry. 